I didn't know it wasn't a widely known day, I thought Boxing Day was Boxing Day everywhere there was a Christmas Day

One theory is to do with the opening of Christmas Boxes - these were what the rich contributed things to, and the boxes were opened by the poor the day after Christmas, presumably because many of the poor worked as servants for the rich, and were therefore required on Christmas Day and wouldn't have had time to open their boxes. In church, the Alms Boxes were contributed to voluntarily by churchgoers and the boxes were opened the day after Christmas and the contents distributed amongst the poor.

I've heard tipping being referred to as a Christmas Box, and I always remember my mother tipping people such as the dustbin men, postman, milkman, newspaper boy/girl and anyone else who did that kind of 'service' throughout the year - they got a Christmas Box, a tip, at Christmas I presume this is what jseal was referring to up here.

Being a Google-aholic, I have just Googled and found quite a good explanation on Snopes. There are a number of theories as to where the name came from, all of which seem feasible to me: click here to learn more about Boxing Day

In Ireland it is referred to as St Stephen's Day, although that is always on 26th December, whereas, in England at least, Boxing Day is moveable to the following Monday or Tuesday, if 26th falls on a weekend.